In our 2006 'bot we used 6WD drive with the KOP tranny and frame. Our robot had its gearbox between the center and rear wheel with two chains, the "interior" one ran from the tranny to the center wheel, then front, then back to the tranny, the "exterior" one went to the center, then rear, then back to the tranny. There is a picture on the robot's FIRSTwiki
entry that might help make sense of my description. The upshot of this layout was, with just two chains, we achieved pretty good redundancy, and any one chain could drop and maintain 5WD (really 4WD) capability, which was handy several times.